Self-regulating tire pressure system and method

ABSTRACT

A self-regulating tire pressure system and method employs a bistable valve (10) that allows air from a high pressure reservoir (6) to replenish the pressure within a tire (4) when it has fallen below an actuating pressure, and discontinues its operation only after the tire pressure has increased to a closing pressure that is greater than the actuating pressure. The system is capable of sensing the valve&#39;s (10) frequency of operation as an indication of a slow tire leak when the frequency exceeds a predetermined threshold, of sensing the number of valve (10) operations and the duration of each operation as an indication of a flat tire condition when the number of operations for a predetermined duration exceeds a second threshold, and of sensing the duration of the valve&#39;s (10) operation as an indication of a low reservoir (6) pressure condition when that duration exceeds a third threshold. The valve (10) includes a bistable diaphragm (94, 120) that receives a reference pressure on one side and the tire pressure on its opposite side, and snaps between two stable positions respectively opening and closing the valve (10) in response to the tire pressure falling below the actuating pressure and then increasing to the closing pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to self-regulating pressure systems and methodsfor vehicle tires, and more particularly to self-regulating systems andmethods in which the tire is automatically replenished from a highpressure reservoir in response to a sensed low pressure condition in thetire.

2. Description of the Related Art

Low tire pressure is an important cause of excessive fuel consumption,tire wear and impaired steerability. A normal tire will typically leakon the order of 25 percent of its pressure per year due to its inherentpermeability. It is thus good practice to check tire pressure on aregular basis. However, even checking tire pressure every few weeks maynot prevent these adverse affects when a slow leak is present, and theleak may go undetected unless a careful record is maintained of howfrequently the pressure in each tire has to be replenished. A fast leakor flat condition can rapidly cause damage to the tire and even renderit unusable in a short period of time, but this condition may gounnoticed by an inexperienced driver until it is too late.

It is thus highly desirable to have some mechanism that automaticallyreplenishes the tire pressure when it is too low, and that warns thedriver of the low pressure condition. One such system is disclosed inU.S. Pat. No. 4,067,376 to Barabino. This patent incorporates a highpressure reservoir into the vehicle wheel, and uses a valve thatautomatically opens a passageway between the high pressure reservoir andthe tire in response to the tire pressure falling below a selectedthreshold level. Operation of the valve also causes a sonic orultrasonic signal to be generated which is detected by a sensor mountedin close proximity to the wheel, typically within the wheel well. Whenit senses a valve operation, the sensor initiates a visual and audiowarning for the driver. To differentiate between the different tires,the valve on each tire may be tuned to generate a signal with a uniquefrequency, or the sensors for the different tires may be tuned to detectdifferent components of a uniform valve signal.

Another system is disclosed in U.S. Pat. No. 4,742,857 to Gandhi. Inthis patent the tire valve includes a magnet that moves in response to afalling tire pressure. Movement of the magnet to a low pressure positionis sensed by a solenoid mounted on the shock absorber adjacent the tire.The magnetic field from the moveable magnet intercepts the solenoid ateach tire revolution, generating a voltage in the solenoid that varieswith the magnet's position. When the magnet's sensed position indicatesa low pressure condition, a warning is provided to the vehicle operator.If the operator wishes to replenish the tire pressure, he or sheactuates a switch that supplies current to the solenoid for theappropriate tire. The solenoid produces a magnetic field that moves itsassociated magnet to a position at which a valve from a high pressurereservoir within the wheel is opened so that air can flow into the tire.

While the above systems can effectively replenish the air pressurewithin a tire, they are incapable of distinguishing between normal longterm leakage from a tire, a slow leak condition that requires repair,and a fast leak or flat tire condition. The vehicle operator is thusuninformed as to whether a tire repair or replacement is called for. Theoperator is also not supplied with any information as to the conditionof the high pressure reservoir, and whether it requires replenishment.Furthermore, the prior use of separate transmitters within each wheeland associated receivers on the vehicle requires a power source withineach wheel and complex electronics, and is unduly expensive.

An after market sensor for low tire pressure has been marketed by EpicTechnologies Inc. The sensor monitors tire pressure and causes a radiofrequency (RF) signal to be transmitted in case the pressure falls belowa predetermined set point; transmission of the RF signal actuates awarning for the vehicle driver. However, the system does not have anymechanism to automatically replenish the low tire pressure.

SUMMARY OF THE INVENTION

The present invention seeks to provide a new self-regulating tirepressure system and method that senses a low pressure condition within atire, automatically replenishes the pressure to a desired level, is lesscomplex and expensive than prior approaches, provides the vehicleoperator with information on the cause of the low pressure condition andwhether tire repair or replacement is required, and does not require theintervention of the vehicle operator to restore tire pressure.

These goals are accomplished with the use of a bistable valve that opensan air flow between the high pressure reservoir and the tire when thetire pressure falls below a predetermined actuating level, and halts theair flow when the tire pressure has increased to a closing pressure thatis greater than its actuating pressure. The valve condition iscommunicated to the vehicle by means of a primary winding on the vehiclechassis adjacent the wheel, and a secondary winding on the wheel that isconnected along with a set of valve contacts in a secondary circuit. Thevalve contacts open and close as the valve switches between its twostable positions, producing a change in the impedance of the secondarycircuit that is reflected back to the primary winding. The reflectedimpedance provide an indication of the valve operations.

A logic circuit supplied by the primary winding differentiates betweenvarious low pressure tire conditions. If the frequency of valveoperation exceeds a predetermined threshold, a slow leak as opposed to anormal pressure loss through the tire wall is indicated. When theduration of the valve operation exceeds a predetermined threshold, a lowreservoir air pressure condition is indicated. When the number of valveoperations that last for a predetermined duration exceeds apredetermined threshold, a flat tire situation is indicated.

The valve preferably incorporates a bistable diaphragm that movesbetween two stable positions respectively opening and closing the airflow passageway from the high pressure reservoir. In one embodiment thediaphragm comprises a flexible membrane that carries a first magneticmember aligned with a stationary magnetic member. At least one of themagnetic members is magnetized to hold the two members together with thediaphragm closing the air flow passageway from the high pressurereservoir when the tire pressure exceeds the valve closing pressure; thediaphragm flexes away from the second magnetic member to its otherstable position and opens the air flow passageway when the tire pressurefalls below the valve actuating pressure. In a second embodiment thebistable diaphragm is a prestressed metal diaphragm that is orientedabout a central plane and has stable positions on opposite sides of thatplane. The metal diaphragm may be lodged in a recess that allows arestricted movement of the diaphragm when it moves between its bistablepositions, thus enhancing the diaphragm's snap action. In thisembodiment an air impermeable flexible diaphragm may be connected tomove with the metal diaphragm, and spans the housing in which the recessis formed to block the flow of air therethrough.

The valve compares the tire pressure with a reference pressure that canbe provided from sources such as a pressure regulator, a spring or asealed bellows. A flow of air from the high pressure reservoir to thetire is actuated when the differential between the compared pressuresexceeds a threshold amount.

Further features and advantages of the invention will be apparent tothose skilled in the art from the following detailed description, takentogether with the accompanying drawings, in which:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a portion of a vehicle tire and wheelillustrating the basic mechanical elements of one embodiment of theinvention;

FIG. 2 is a block diagram of the invention;

FIG. 3 is a diagram of a tire and wheel illustrating theirinterconnection through the bistable valve used in the invention;

FIG. 4 is a block diagram showing the communication between the vehicleand wheel to detect when the valve has operated;

FIG. 5 is a logic diagram of the logic circuitry used to distinguishbetween various types of tire leak and low reservoir conditions;

FIG. 6 is a graph comparing the pressure in the high pressure reservoirwith the time required to restore the tire pressure when a pressureregulator is used to provide a reference pressure for the valve;

FIG. 7 is a sectional view of a bistable valve construction that employsa pressure regulator;

FIG. 8 is a sectional view of a bistable valve construction that employsboth a pressure regulator and a spring as a combined reference pressuresource;

FIG. 9 is a sectional view of an alternate bistable valve constructionthat employs a sealed bellows as a reference pressure source;

FIG. 10 is a graph comparing the pressure in the high pressure reservoirwith the time required to restore the tire pressure when a pressureregulator is not employed; and

FIG. 11 is a fragmentary sectional view of a valve and high pressurereservoir incorporated into a conventional wheel.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, the relevant elements of a vehicle wheel 2with a tire 4 seated thereon are illustrated. The wheel 2 includes abuilt-in high pressure reservoir 6, with a valve stem 8 extending intothe reservoir so that it can be replenished with pressurized air. Thesystem is preferably designed to operate with a reservoir pressure ofover 100 psi, which is commonly available at most filling stations; 150psi is preferred.

A valve 10 provides an air passageway between the high pressurereservoir 6 and the interior of the tire 4. In accordance with theinvention, the valve 10 has a bistable construction such that it opensthe passageway between the high pressure reservoir and tire when thetire pressure has dropped below a predetermined threshold level,allowing air from the reservoir to flow into the tire and restore itspressure. The valve does not close off the passageway until the pressurewithin the tire has reached an elevated level that exceeds the originalactuating pressure. For example, the valve may automatically actuatewhen the tire pressure falls below about 34 psi, but not reset until thetire pressure has reached a higher level such as 35 psi.

The opened/closed status of the valve 10 is communicated to the vehicleby means of a coil of wire 12 installed on the outer periphery of thewheel. A set of contacts within the valve 10 open when the valve s inone state and close when the valve is in its other state, and areconnected in series with the winding 12 via a pair of wires 14. Theoverall winding circuit is open When the valve contacts are open, andclosed when the contacts are closed.

Mounted on the wheel well 16 of the vehicle chassis approximatelyonequarter inch from the wheel windings 12 is a primary winding coil 18that senses the reflected impedance of the secondary winding circuit 12,and thus determined whether the valve 10 is open or closed. The primaryand secondary windings 18 and 12 preferably comprise 40 turns of 20gauge and 6 turns of 17 gauge solid copper magnet wire, respectively. Apair of lead wires 20 provide the input to primary winding 18, and alsocouple the reflected impedance signal from the primary winding 18 tologic circuitry described below. Similar connections are made to theprimary windings which communicate with secondary windings on each wheelseparately, so that the status of each tire can be displayed to theoperator separately.

FIGS. 2 and 3 show additional aspects of the overall system. Thesecondary winding 12 is shown connected in series with pair of switchcontacts 22 that are operated by the bistable valve 10, while theprimary winding 18 provides a signal to the system's control electronics24 to indicate the valve's status. The control electronics sense thefrequency and duration of the valve operations, interpret this data todetermine the nature of a low pressure condition, and actuate thevehicle dashboard display 26 to notify the vehicle operator of thenature of a problem with any of the tires, and the identity of theaffected tire.

The high pressure reservoir 6 supplies a reference pressure source 28such as a pressure regulator; a sealed bellows or a spring can also beused to establish the reference pressure. The reference pressure source28 normally has a single set point at which it tries to maintainpressure. With the addition of the bistable valve 10, a second and lowerset point determined by the valve is added. For example, assume that foran automobile tire it is desired to begin replenishing the tire pressurewhen it drops below 34 psi, and to continue increasing the tire pressureuntil it reaches 35 psi. In this situation the reference pressure source28 does not begin correcting the tire inflation until the lower setpoint (34 psi) is reached. At that time the valve is actuated and allowshigh pressure air from reservoir 6 to replenish the tire pressure. Whenthe tire pressure has been restored back to its normal set point (suchas 35 psi) the valve connection automatically closes, therebydiscontinuing further filling of the tire. The reference pressureprovided to the valve for a 150 psi high pressure reservoir 6 may beabout 36 psi, for example.

A block diagram of circuitry that is suitable for indicating the valve'sstatus, and thus whether a leak in the tire is present, is given in FIG.4. A low duty cycle waveform, preferably with a duty cycle substantiallyless than 50%, is generated by an oscillator 30. The waveform ispreferably generated as a series of pulses at a frequency of a few Hz toabout 20 Hz for low power applications, with a pulse duration of about20-25 microseconds. The pulses operate a circuit driver 32, preferablyimplemented as a PNP darlington transistor. The output pulses fromdriver 32 are fed through the primary winding 18 on the vehicle chassis,and are coupled thereby to the secondary winding 12.

The impedance of the secondary circuit that includes secondary winding12 is reflected back to the primary winding 18 to provide an indicationof whether the valve contacts 22 are open or closed, and thus whetherthe valve is operating to restore the tire pressure or is quiescent. Thereflected impedance appears as a small load when the contacts are open,and as a high load when the contacts are closed. This can be implementedin several ways. One method is to provide constant current pulses to theprimary and look for a reduction in secondary voltage when the contactsare closed. Another method is to provide constant voltage primary pulsesand look for an increase in secondary current as an indication of closedcontacts. A third method is to connect a capacitor 34 in parallel withthe secondary winding 12 and contacts 22. The capacitor 34 together withthe inductance of secondary winding 12 forms a tuned circuit thatoscillates at a resonant frequency when the contacts 22 are open, but isshort-circuited when the contacts 22 are closed. An open contactcondition, reflecting an operation of the valve, produces a "ringing" orresonant oscillation when the core of the primary winding 18 has becomesaturated and the input pulse turns off.

The waveform reflected back to the primary winding 18 is fed through aclamping circuit 36 to a zero crossing comparator circuit 38. Thecomparator circuit responds to ringing signal by squaring up its edgesand presenting a series of pulses to a counter 40, the operation ofwhich is enabled when the oscillator 30 is operating by means of a latchcircuit 42 fed from the oscillator. The counter 40 functions as ahardware filter, requiring a fixed number of pulses from the comparator38 before it produces an output. For the primary/secondary circuits anddrive signal described above, five input pulses could be required toproduce an output from counter 40. This output triggers a one-shot latch44 that operates a driver 46 to turn on a low tire pressure indicatorsuch as light emitting diode 48 on the vehicle's display panel.

The circuitry of FIG. 4 simply indicates that the bistable pressurevalve has operated, without identifying the tire condition that causedthe valve operation. A logic circuit that responds to the signalreflected from the secondary winding circuit back to the primary windingby distinguishing between different low pressure conditions is shownFIG. 5. It utilizes a clock 50 that generates pulses at a desired rate,such as 1 Hz. The status of the bistable valve, as indicated by theimpedance reflected back to primary winding 18, is fed from an inputline 51 into a two-bit counter 52 that operates by producing a Q1 outputover output line 54 the first time the valve operates, and both Q1 andQ2 outputs respectively over output lines 54 and 56 the second time thevalve operates.

The output of clock 50 and the Q1 output of the two-bit counter 52 areconnected respectively to the clock and enable inputs of a timingcircuit 58. The timing circuit determines the time interval between twosuccessive valve operations. If this interval is less than apredetermined threshold duration, a slow leak as opposed to a normallong term loss of tire pressure is indicated. For example, the timingcircuit 58 is shown implemented as an 18-bit counter which produces anoutput after running for 262,144 seconds, or approximately 3 days (the18-bit counter is fabricated by connecting readily available 14-bit and4-bit counters in series). A Q (high) output is produced by counter 58when it has counted up to its limit.

The Q2 output of 2-bit counter 52 is connected to a latch 62 thatoperates a "slow leak" lamp 64 on the vehicle's dashboard panel. The Qoutput of the timing circuit counter 58 is connected through an OR gate66 to reset both of the counters 52 and 58; a manual counter reset isalso provided through OR gate 66.

The operation of the FIG. 5 circuit in producing a slow leak warningwill now be described. Assume first that a slow leak is present, causingsuccessive valve signals to be generated at intervals of less than 3days. The first valve signal causes 2-bit counter 52 to enable theoperation of timing circuit counter 58, which begins to count up. Thesecond valve signal appears before counter 58 has reached its limit.This second signal activates both the Q1 and Q2 outputs of 2-bit counter52, which respectively continue the enablement of timing circuit counter58 and provide an input to latch 62, causing the "slow leak" lamp 64 toilluminate.

Assume next that a second valve operation is not detected before thetiming circuit counter 58 has reached its limit. In this situation theQ2 input to latch 62 remains low while the timing circuit counter 58counts all the way up to its limit, preventing the "slow leak" lamp 64from lighting. A Q output is produced by timing circuit counter 58 whenit reaches its limit, causing both the counter 58 and the 2-bit counter52 to reset. The next valve signal to be received will thus appear as aninitial signal that enables a restart of the logic circuit operation,with the "slow leak" lamp 64 continuing to be held off unless anadditional valve signal is subsequently received before the timingcircuit counter 58 has counted out.

While the circuit described thus far in connection with FIG. 5 has beenreferred to as a "slow leak" circuit, it does not distinguish between aslow leak and a fast leak or flat tire situation. A fast leak can bedetected with a similar logic circuit that uses a lower capacity timingcircuit counter, for example an 8-bit counter that lights a "fast leak"indicator for successive valve operations in less than 4 minutes.Operation of the "fast leak" detection circuit can be used to lock outthe "slow leak" detector for that tire. Alternately, the manual resetinput 68 to OR gate 66 can be used to distinguish a fast from a slowleak. If the operator observes the "slow leak" lamp, he or she canoperate the manual reset to extinguish the lamp. If it comes on again ina short period of time, a fast leak is indicated.

Several different tire and high pressure reservoir conditions can bedetected by observing the duration of the valve operations, a capabilitythat is provided by the present bistable valve approach. To determinethe duration of valve operations, a counter such as 8-bit counter 70receives a clock signal from the 1 Hz clock 50, and both enable andreset signals from the valve input line 51. This configuration causesthe counter 70 to count up when both clock and enable signals arepresent, and to immediately reset when the enable signal is removed.Counter 70 determines the duration of each valve operation. Its seventhand eighth output bits, respectively Q7 and Q8, are connected as inputsto AND gates 72 and 76, respectively. The valve signal line 51 isconnected as a second input to both of these AND gates. Thus, AND gate72 will produce an output when the Q7 output of output of counter 70goes high and a valve signal is present, and AND gate 76 will produce anoutput when the Q8 output is high and the valve signal is still present.The Q7 and Q8 counter outputs correspond to valve operations of 128 and256 seconds, respectively.

A representative relationship between the air pressure within the highpressure reservoir and the duration of the valve operation, when aregulated reference pressure is compared with the tire pressure toactuate a flow of air from the reservoir to the tire, is illustrated inFIG. 6. Since air is bled from the high pressure reservoir during eachvalve operation, its pressure will progressively decrease for multiplevalve operations, or if one valve operation lasts for an extended periodof time. The reduction in reservoir pressure as a function of theaggregate time the valve has operated, either as a result of multipleoperations or from a single extended operation, is indicated by curve78. The rate of pressure reduction in the reservoir is a function of theinitial reservoir pressure, the valve set point pressures, and therelative volumes of the reservoir and tire. To provide the operator withan indication that the pressure within the reservoir has been reduced toa point at which it should be replenished, a "reservoir low" warning canbe provided at a somewhat arbitrary selected pressure, such as 50 psi.In addition, extended mobility tires are presently contemplated that cancontinue to be operated for up to several hundred miles after a flat hasbeen incurred; a warning can also be provided if this occurs.

These various situations are indicated in FIG. 6. When the reservoirreaches an undesirably low pressure, such as 50 psi, the valve operatingtime necessary to replenish the tire pressure increases to T1. For aflat in an extended mobility tire the valve will repeatedly operate,with the tire gradually losing pressure and the reservoir being furtherdepleted with each operation; the duration of each operation will remainrelatively constant at T2, until a pressure of about 1 psi is reached(when a 1 psi pressure differential is used to actuate the valve).Thereafter the valve will remain open with little or no furtherreplenishment of tire pressure.

Referring back to FIG. 5, a "reservoir low" situation is displayed by alamp 78 that is actuated by a latch 80 when both the Q7 and the valvesignals are presented to AND gate 72. To display a flat tire warning, a3-bit counter 82 connected to the Q7 output of 8-bit counter 70 producesan output when counter 70 has counted up to Q7 (128 seconds) eightseparate times. This corresponds to eight valve operations of durationT1 (FIG. 6), indicating that the tire pressure has fallen to less than20 psi. The output of 3-bit counter 82 is delivered through an OR gate88 to a latch 90 that lights a "flat tire" lamp 92. The flat tire lamp92 is also illuminated by a single occurrence of the Q8 (256 seconds)output of counter 70, processed through AND gate 76 and OR gate 88. Evenif the 3-bit counter 82 has not yet lit the flat tire lamp 92, theoccurrence of a valve operation for 256 seconds or more can be taken asan indication of a flat tire.

Only a few indicators that may be obtained from the informationregarding valve operating frequency and operating duration have beendescribed. Numerous other warning indicators that take advantage of thebistable valve employed by the invention may also be envisioned. Itshould also be realized that the assignment of specific times such as128, 256 or 512 seconds to identify particular situations is somewhatarbitrary, and can be adjusted as necessary to meet the needs of anyparticular tire/wheel system.

One implementation of a suitable bistable valve 10 is shown in FIG. 7. Aflexible diaphragm 94 spans a compartment 96 within the valve housing,segregating the pressure on one side of the diaphragm from that on theother side. A permanent magnet 98 is affixed to a striker 99 clamped tothe center of the diaphragm, and is aligned with a bore 100 formed in avalve end piece 102 of magnetic material. A regulated pressure of say 36psi is supplied from a pressure regulator 103 to the end piece 102through a conduit 104. The pressure regulator 103 is supplied with airfrom the high pressure reservoir via a conduit 105, with a branchconduit 106 supplying the same reservoir pressure to the opposite end ofthe valve housing. A valve stem 108 on the opposite side of diaphragm 94from the end piece 102 receives the reservoir pressure from conduit 106on one side, and on its opposite side has an actuating tip 110 that isaligned with the diaphragm striker 99 within compartment 96. Thecompartment 96 is vented to the tire through an outlet 112, and issegregated from the reservoir pressure of conduit 106 by a flange 114that retains the valve stem 108 in place.

The diaphragm 94 thus receives the regulated pressure on its right handside, and the tire pressure on its left hand side. The diaphragm, magnet98 and valve end piece 102 are selected so that the magnet remainsseated against the end piece 102 for a tire pressures greater than thedesired valve actuating pressure. This establishes one of the valve'stwo stable conditions. When the tire pressure falls below the actuatingthreshold, say 34 psi, the increased pressure differential across thediaphragm causes it to flex and begin to lift the magnet 98 away fromthe valve end piece 102. The magnet's movement away from the valve endpiece in turn reduces the magnetic force of attraction between these twoelements, thus accelerating the movement of the diaphragm to the left.As the diaphragm continues to travel to the left, but before it reachesit center position, it pushes against the valve stem's actuating tip110, opening an air passageway through the valve stem that allowsreservoir air from conduit 106 to flow through the valve stem and outlet112 to the tire. The valve stem remains open until the tire pressure hasincreased to the valve's second set point, for example 35 psi. At thispoint the pressure differential across the diaphragm is reduced to 1psi, and the residual magnetic force between the permanent magnet 98 andthe magnetic end piece 102 causes the diaphragm to lift off theactuating tip 110, terminating the flow of air into the tire. Since thediaphragm is now moving towards the end piece 102 the magneticattraction between magnet 98 and the end piece increases, causing thediaphragm to snap over to the right to a stable off position with themagnet clamped to the end piece 102.

The valve is thus truly bistable, with only two stable positions. In oneposition the valve is fully off, with the magnet 98 clamped against theend piece 102. In the other stable position the valve is fully open,with the striker 99 depressing the valve stem actuating tip 110.

The metallic end piece 102 and the magnet 98 form the contacts 22 thatwere described in connection with FIGS. 2 and 4, and that are connectedin the secondary circuit to indicate whether the valve is open orclosed. Electrical lead wires 116 and 118 are connected respectively tothe end piece 102 and to the metallic striker 99 for this purpose. Whenthe magnet 98 is clamped against the end piece 102 in the position shownin FIG. 7, the contacts are closed and the capacitor 34 of FIG. 4 isshort-circuited, thus preventing a ringing in the secondary circuit andproviding an indication that the valve is off. When the diaphragm hasflexed to the left and is depressing the valve stem actuating tip 110,the magnet 98 is electrically isolated from the end piece 102; thisopens the contacts 22 of FIG. 4 and produces a resonant LC secondarycircuit that indicates the valve is operating.

It is important that the valve have no stable position other than fullyopen or fully closed, so that it will not continuously supply a smallsteady air flow to a slow leak without cycling. Another valvearrangement that can also be used for this purpose is illustrated inFIG. 8. Several elements of this embodiment are the same as for thevalve of FIG. 7, and are identified by the same reference numerals. Inthe embodiment of FIG. 8 a prestressed metal diaphragm 120 has aperipheral lip 122 that is clamped to the valve housing so that thediaphragm spans the interior of the housing. An air impermeable flexiblediaphragm 124 spans the housing adjacent to the metal diaphragm 120,with its periphery clamped in place so that air cannot flow from oneside of the rubber diaphragm to the other. The tire pressure isintroduced into the compartment 96 on the valve side of the airimpermeable diaphragm 124, while regulated pressure from the pressureregulator (not shown) is introduced into a spring compartment 126 on theopposite side of the metal diaphragm 120 through an inlet port 127, andhigh pressure air from the reservoir is introduced to a high pressurearea 128 at the valve inlet. A metallic connector 129 clamps the middlesections of the two diaphragms and forces them to flex together. Anelectrical connection is made to the valve stem tip 110 through thevalve stem 108 via a metal collar 130 around the valve stem and a leadwire 131 that extends through a feed-through bushing 132 in the housingwall. An electrical connection is made to the metal diaphragm 120 with alead wire 134 that connects to the valve housing at any convenientlocation. Lead wires 131 and 134 are connected in series with thesecondary winding on the wheel.

A coil spring 136 is lodged within the spring compartment 126 between alip on the metallic connector 129 and the opposite end of the springcompartment 126. The spring 136 provides a reference pressure thatopposes the tire pressure on the opposite side of the diaphragms 120 and124 to control the valve operation. While the spring 136 might be usedas the sole source of reference pressure, the spring force tends beunevenly distributed across the metal diaphragm 120, and the magnitudeof the spring force varies with the diaphragm position. These drawbacksare mitigated with the addition of the regulated air pressure within thespring compartment 126, which supplies an evenly distributed pressure tothe diaphragm that does not vary with the diaphragm position. When theregulated pressure is used together with the spring to provide areference pressure the spring pressure may be correspondingly reduced,such as to about 20 psi.

In the position shown in FIG. 8, the dome of the prestressed metaldiaphragm 120 is flexed to the left to depress the valve stem tip 110and actuate the valve stem. High pressure air thus flows into the tireuntil the tire pressure has increased to the desired second set point,at which time the reduced pressure differential across the diaphragmcauses it to snap to its second stable position (indicated by a dashedline), with its dome flexed to the right away from the valve stem. Thetwo lead wires 131 and 134 are electrically connected when the diaphragm120 is flexed leftward to actuate the valve, and electrically separatedwhen the diaphragm is flexed rightward to shut the valve off. The makingand breaking of these contacts provides an indication of the valvestatus, in a manner similar to (but out of phase with) the contacts ofFIG. 7.

The prestressed metal diaphragm 120 has only 2 stable positions, withits dome flexed either left or right as indicated in FIG. 8. The metaldiaphragm is preferably formed from 0.005 inch thick stainless steel,1.25 inch in diameter with a 0.04 inch high dome.

Another valve embodiment that employs a different source of referencepressure is illustrated in FIG. 9. In this embodiment a sealed bellows138 is filled to the desired reference pressure, such as 36 psi. Abistable metal diaphragm 140 similar to the diaphragm 120 of FIG. 8 hasits center connected to move with the bellows 138 between valveactuating and valve non-actuating positions. As shown in FIG. 9, thediaphragm 140 has a central opening that is mounted on a striker 142affixed to the bellows. When flexed to the right as shown, the valvestem 108 is not actuated. When the tire pressure outside the bellowsdrops below the valve actuating point, the reference pressure within thebellows causes the diaphragm to snap to the left, depressing valve stemtip 110 to actuate the valve. This permits high pressure reservoir airfrom the left side of the valve stem 108 to flow through the valve andout to the tire. The position of the valve is monitored by the lead wire131 connected to the valve stem tip 110 as in FIG. 8, and by a secondlead wire 144 that is either connected to the striker 142 through ametallic bellows end plate 146, or through the metallic diaphragm 140 inthe valve housing if a good electrical connection is maintained betweenthese two parts.

The prestressed metal diaphragm 140 is similar to the diaphragm 120 ofFIG. 8, but it is shown lodged within an inner peripheral recess 148formed in the wall of the valve housing. The diaphragm thickness is lessthan the width of the recess, allowing the diaphragm to "float" axiallywithin the recess. This adds to the diaphragm's snap action when itmoves from one of its stable positions to the other.

A generalized curve that relates the pressure in the high pressurereservoir to the valve operating time required to replenish the tirepressure, for a valve in which a sealed bellows or a spring but not aregulated pressure is used as a reference for the tire pressure, isillustrated in FIG. 10. Since the pressure supplied by a sealed bellowsor spring reference will not fall below its predetermined set point,even when the pressure in the high pressure reservoir falls below thatset point as a result of repeated valve operation, the valve will bepermanently actuated once the high pressure reservoir falls to thepressure st point for resetting the valve after an operation. This isindicated in FIG. 10 by a flattening of the curve so that it becomeshorizontal once the illustrative set point of 35 psi is reached. Eventhrough the reservoir pressure will fall below 35 psi if the tirecontinues to leak, the reference pressure will remain at its presetlevel, such as 36 psi. For this type of valve the flat tire warninglogic of FIG. 5 will not be applicable, although the other warningsshown in FIG. 5 can still be provided.

FIG. 11 shows the valve of FIG. 9 incorporated into a wheel 2. Amounting plate 148 is welded across two existing sections of the wheelrim, with the valve 150 carried by the mounting plate within a spacebetween the two wheel sections. Additional plates 152 and 154 are weldedacross contours around the periphery of the wheel to form reservoirsections that are interconnected by drilled openings 156. The primarywinding 18 is shown wound around a core 158, with the processingelectronics mounted on a substrate 160 inside a molded polyurathanehousing 162. The invention can thus be incorporated into a standardwheel, although customized wheels may also be provided.

A tire pressure regulating system and operating method that providesdetailed information on the exact condition of the tire, and is bothsimple and inexpensive in construction, is made possible with theinvention. While several illustrious embodiments of the invention havebeen shown and described, numerous variations and alternate embodimentswill occur to those skilled in the art. For example, the valvesdescribed herein will have applications outside of tire pressureregulation, and the secondary winding contacts can be operated normallyclosed rather than normally open. Such variations and alternateembodiments are contemplated, and can be made without departing from thespirit and scope of the invention as defined in the appended claims.

We claim:
 1. A self-regulating tire pressure system, comprising:a wheelfor seating a tire, said wheel including a reservoir for providing ahigh pressure air source, and a bistable valve establishing an air flowcommunication between said reservoir and a tire seated on said wheel,said valve having a first stable position in which said valve opens anair flow passageway between said reservoir and said tire in response tothe air pressure within the tire falling below a predetermined actuatingpressure set point, and having a second stable position in which saidvalve closes said air flow passageway in response to the air pressurewithin the tire increasing to a predetermined closing pressure set pointwhich is greater than said actuating pressure set point.
 2. Theself-regulating tire pressure system of claim 1, said bistable valveincluding a reference pressure source and a bistable diaphragm forreceiving said reference and tire pressures on opposite sides of thediaphragm, said diaphragm having first and second stable positionsrespectively opening and closing said air flow passageway in response tosaid tire pressure respectively falling below said actuating pressureand increasing to said closing pressure.
 3. The self-regulating tirepressure system of claim 2, said bistable diaphragm comprising aflexible membrane carrying a first magnetic member, said valve furthercomprising a second magnetic member aligned with said first magneticmember, at least one of said magnetic members being magnetized to holdsaid magnetic members together with said diaphragm in said second stableposition when said tire pressure exceeds said closing pressure, and toallow said diaphragm to flex away from said second magnetic member toits first stable position in response to said tire pressure fallingbelow said actuating pressure.
 4. The self-regulating tire pressuresystem of claim 2, said bistable diaphragm comprising a prestresseddiaphragm oriented about a central plane and having stable positions onopposite sides of said central plane.
 5. The self-regulating tirepressure system of claim 4, said reference pressure source comprising asealed bellows with an internal pressure establishing said referencepressure, said sealed bellows bearing against said diaphragm inopposition to said tire pressure.
 6. The self-regulating tire pressuresystem of claim 4, said reference pressure source comprising amechanical spring bearing against said diaphragm in opposition to saidtire pressure.
 7. The self-regulating tire pressure system of claim 1,further comprising means for sensing said valve's frequency of operationas an indication of a slow tire leak when said frequency exceeds apredetermined threshold, said means including a secondary windingcircuit mounted on the outer periphery of said wheel and a primarywinding circuit mounted on the vehicle chassis for sensing the reflectedimpedance of said secondary winding circuit and a circuit that respondsto the reflected impedance signal from the primary winding circuit toindicate operation of said valve.
 8. The self-regulating tire pressuresystem of claim 7, further comprising a logic circuit that responds tothe reflected impedance signal from the primary winding circuit bysensing the number of valve operations and the duration of each suchoperation as an indication of a flat tire condition when the number ofsaid operations for a predetermined duration exceeds a predeterminedthreshold.
 9. The self-regulating tire pressure system of claim 1,further comprising means for sensing the duration of said valve'soperation as an indication of a low reservoir air pressure conditionwhen said duration exceeds a predetermined threshold.
 10. Theself-regulating tire pressure system of claim 1, for a vehicle havingsaid wheel mounted with respect to a vehicle chassis, further comprisingsensing means including:a primary electrical winding on said chassisproximate to said wheel, means for delivering an alternating electricalsignal to said primary winding to establish a primary winding fluxfield, a secondary winding on said wheel within said primary windingflux field, a valve indicator means connected in circuit with saidsecondary winding for varying the secondary winding impedance reflectedback to said primary winding according to whether said valve isestablishing said air flow communication, and timing means responsive tosaid reflected secondary winding impedance for timing operations of saidvalve.
 11. The self regulating tire pressure system of claim 10, whereinsaid means for delivering an alternating electrical signal to saidprimary winding delivers a pulsed signal with substantially constantcurrent pulses, and said timing means is responsive to the voltageacross said primary winding.
 12. The self-regulating tire pressuresystem of claim 11, wherein said means for delivering an alternatingelectrical signal to said primary winding delvers a pulsed signal withsubstantially constant voltage pulses, and said timing means isresponsive to the current through said primary winding.
 13. Theself-regulating tire pressure system of claim 10, wherein said secondarywinding and valve indicator means comprising a tuned circuit withrespect to the alternating electrical signal on said primary windingwhen said valve is establishing said air flow communication, and saidtiming means is responsive to a tuned resonance between said primary andsecondary windings.
 14. The self-regulating tire pressure system ofclaim 10, said means for delivering an alternating signal to saidprimary winding delivering a signal with a duty cycle substantially lessthan 50%.
 15. A self-regulating tire pressure system, comprising:a wheelfor seating a tire, said wheel including a reservoir for providing ahigh pressure air source, and a bistable valve establishing an air flowcommunication between said reservoir and a tire seated on said wheel,said valve opening an air flow passageway between said reservoir andsaid tire in response to the air pressure within the tire falling belowa predetermined actuating pressure, and closing said air flow passagewayin response to the air pressure within the tire increasing to apredetermined closing pressure which is greater than said actuatingpressure, said bistable valve including a reference pressure source anda bistable diaphragm for receiving said reference and tire pressures onopposite sides of the diaphragm, said diaphragm having first and secondstable positions respectively openings and closing said air flowpassageway in response to said tire pressure respectively falling belowsaid actuating pressure and increasing to said closing pressure, saidbistable diaphragm comprising a prestressed diaphragm oriented about acentral plane and having stable positions on opposite sides of saidcentral plane, said valve including a housing with an inner peripheralrecess, said prestressed diaphragm being lodged in said recess to allowa restricted movement of said diaphragm within said recess when saiddiaphragm moves between bistable positions on opposite sides of itscentral plane, and further comprising an air impermeable flexiblediaphragm spanning said housing to block the flow of air therethrough,said flexible diaphragm being connected to move with said prestresseddiaphragm.
 16. A self-regulating tire pressure system, comprising:awheel for seating a tire, said wheel including a reservoir for providinga high pressure air source, and a bistable valve establishing an airflow communication between said reservoir and a tire seated on saidwheel, said valve opening an air flow passageway between said reservoirand said tire in response to the air pressure within the tire fallingbelow a predetermine actuating pressure, and closing said air flowpassageway in response to the air pressure within the tire increasing toa predetermine closing pressure which is greater than said actuatingpressure, said bistable valve including a reference pressure source anda bistable diaphragm for receiving said reference and tire pressures onopposite sides of the diaphragm, said diaphragm having first and secondstable positions respectively opening and closing said air flowpassageway in response to said tire pressure respectively falling belowsaid actuating pressure and increasing to said closing pressure, saidreference pressure source comprising a pressure regulator for providingsaid reference pressure from said high pressure reservoir at apredetermined regulated pressure.
 17. A self-regulating tire pressuresystem, comprising:a wheel for seating a tire, said wheel including areservoir for providing a high pressure air source, a valve forestablishing an air flow communication between said reservoir and a tireseated on said wheel in response to a low air pressure condition in thetire, and means for sensing the timing of operations of said valve as anindication of the type of low pressure condition, wherein said valve isa bistable valve having a reference pressure source establishing areference pressure and wherein the bistable valve has first and secondset points, said first set point being at a lower pressure than thesecond set point, and open and closed positions respectively at firstand second set points for establishing said air flow communication inresponse to said tire pressure falling to said first set point, andbreaking said air flow communication in response to said tire pressureincreasing to said second set point.